An investigation of the effects of some tracts of supraspinal origin on interneurons of the ventral horn of the lumbosacral spinal cord and the convergence of effects from peripheral afferents on these cells will be made. Since a large part of the effect of the pathways controlling motor activity appears to be exerted through interneurons rather than directly onto motor neurons, it is important to know more about the function of these cells. Many of these interneurons are probably involved in other neuronal circuits. Therefore the input to these cells from peripheral afferents will be tested with regard to the convergence with supraspinal tract fibers. In cats anesthetized with pentobarbital or alpha-chloralose, the origins of the medullary and pontine reticulospinal tracts and the lateral vestibulospinal tract will be stimulated in the brain stem with metal electrodes. Laminae VII and VIII of the lumbosacral spinal cord will be searched for interneurons which respond to the stimulation of these tracts. The location of each interneuron from which records are made will be determined by their depth in the electrode tracks from sections of the cord or by injecting a fluorescent dye.